Do you use Offline live code comparison tool?

[u/karthikjpt]

Last month, I found myself in a tough spot. I was working on three separate HTML and JavaScript pages that shared almost identical logic, but differed in layout, alignment, and specific datetime formatting. Because they used slightly different approaches for the same underlying functions, I couldn't simply merge them or copy everything blindly.

I needed to compare them line by line across roughly 5,000 lines per file (15,000 lines total) and organize those blocks into a unified structure.

To make matters harder, my development setup is extremely minimal: just a 15-year-old laptop, a basic text editor (no VS Code or heavy IDEs), me, and AI.

The Problem with Manual Comparison

Doing a line-by-line manual check across 5,000 lines in three different files is exhausting.

High Risk of Errors: A single copy-paste mistake means spending the whole day hunting for what went wrong.

Lack of Undo Safety: Constant power cuts make manual tracking even riskier.

Zero Visual Feedback: Without a side-by-side visual reference, it is far too easy to miss code blocks during repetitive tasks.

The Solution: Build Your Own Tool

Instead of suffering through it, I decided to build a lightweight comparison tool from scratch. It took me half a day to put together a row-by-row visual comparator using a main reference file.

Once the tool was ready, organizing all 15,000 lines across the three files took only a few hours. I then used AI to clean and optimize the code. In the end, I successfully organized 15,000 lines of frontend code and 8,000 lines of backend code, vastly improving overall performance in record time.

Turning It Into a Public Product

Encouraged by how well it worked, I spent the last few weeks turning that utility into a production-grade tool designed for the public.

I built it with a few core principles in mind:

100% Offline Capability: Powered by modern web capabilities (like Service Workers), once you visit the site, you can use it completely offline with zero feature loss—even if your internet drops.

Privacy First: Data stays secure on your device and is never pushed to a remote server.

Flexible Storage & Portability: You can easily switch to cloud sync or export/import everything so you never lose your progress.

No Installation Required: Perfect for older or restricted hardware where installing heavy desktop software isn't an option.

Market Validation & Looking Ahead

Running a public platform requires servers, domains, and ongoing costs. Given my current financial situation, I want to gauge real demand before taking the leap.

Current market alternatives often charge $20+ per month and frequently require heavy software installations.

I’d love to hear your thoughts on this:

Would you use a lightweight, fully offline, easy-to-customize tool where you can export/import everything without losing progress across devices?

How much would you realistically be willing to pay per year for a tool like this?

[u/-svde-]

did you even bother looking for existing tools?

what do you mean 15k lines took your program a few HOURS? it takes most tools … seconds. instant feedback. how could it possibly take a tool hours to parse a text diff? do you mean you actually built a manual comparator that you then used to go line by line? why? 3 way merge tools exist. and then you still used AI to “clean it up” anyway?

let me save you time, since this apparently took you multiple weeks (even though there’s no way that’s true): no one will pay for this. every single major IDE, text/code editor, git client, and many other types of devtools have robust, fast, and comprehensive text diff tools built in for free, not to mention the countless FOSS utilities that exist online.

not a single person is going to pay you money, let alone a monthly subscription, for a vibe coded single purpose diff tool that also requires manual comparison. seriously do even 2 minutes of market research before you waste more time and money.

[u/karthikjpt → u/-svde-]

Initial search for existing tools showed me that exiting online tools are input process and output method. No Live editing. Also lacks block by block comparison, hide matching, showing unique which is important for tracking all three files progress and targeting mismatch in 3000 lines with single switch. Text diff takes happens in real time... I took only few hours to organise all 20000 lines.

I am not professional developer/ just vibe coding so I don't know most of exiting tools and how they work. That's why this post I posted here to learn from others.

3 way merge is basically merging two files into base file i am right? But tool just used for code refactoring and structural alignment like whitespace adjustment.

Thank you so much for making me aware about market situation. I well aware professional workflow is very different. I also heard client side they don't allow install software in their computer if you want do code comparison in other computer than yours. Also I am focusing on vide coder who copy and paste between project and LLM might need reliable online comparison tool before pasting into project so they can understand what exactly they going to paste and differentiation between given code and response from AI.

[u/-svde- → u/karthikjpt]

first of all, every single one of the features you mentioned are standard in most of these programs. idk what searching you actually did but clearly it wasn't much.

why in the world would you spend multiple hours "organizing" the lines manually? that makes literally no sense. no one, professional or amateur or 100% pure vibe coder, is doing this in a manual comparator.

also i'm sorry but is your tool supposed to be 100% local-only/offline, private, secure utility, or an online web service? you understand that this is completely contradictory, right?

no, 3 way merge is not merging two files into a base file. it's taking a common base file and 2 diverging branches and cross compositing them to combine changes and report conflicts based in the nearest common ancestor principle.

used for code refactoring and structural alignment like whitespace adjustment

^ this is NOT the job of a diff tool. it's a basic structure built into all IDE/text programs or done with specific tools like linters, Prettier, or other opinionated formatters.

every other sentence contradicts the one before it lol. you are "well aware" pro workflow is different yet have no idea what pros actually use. or anyone, for that matter. what "client side" people are you talking about? employees of businesses? most employees who would need something like this either have a program that does it already, can get one approved, or will just use one of the many free online tools that do everything you described.

you didn't post here "to learn" you posted here to try to sell a worse standalone version of an industry standard tool built into the systems all people already use. on a subscription basis. i hardly think you can market this as a "reliable tool" when you don't know the difference between diffing, linting, formatting, and so on and can't even be honest about market research.

[u/karthikjpt → u/-svde-]

Thanks for sharing some keywords to learn. Yes, learning to build tool that can be sold. I checked all top results in Google with keyword searching "text comparison tool". They are doing same function but differently. You can see difference but can't edit them live. You have to edit them by looking at it in your own file and again upload to check. Or output in another panel. But mine 100% offline capabilities with live editing ... You all have to visit once website then will work 100% without internet so you next time visit site my app already there for you without internet. It's helpful if you going to use only notepad without internet. I always thought to install install VS code but I was afraid my old machine can't handle it. So doing web development with just browser and notepad. So I never thought about installing coding tools... My setup already very lightweight which can perform very fast in 15 years old laptop. I can't go back to windows life which was my 12 years slow life without learning about computer life gone. Now living my life nothing less compared to what professional builds. but only with notepad and browser... That's why I build this text comparison tool to compare LLM code before pasting into project. I will looking into integration along with famous LLM like gemini and Chatgpt so with simple tab on browser extension can compare given and returned code difference.

[u/traderprof]

the offline part is useful, but i would pay for it only if it helps decide which difference is intentional. raw diffs are cheap. keeping the reason behind a change is the hard bit.

[u/karthikjpt → u/traderprof]

Are telling from 3 way merge lo like professional looking at changes but don't know reason? That's means you need AI capabilities to describe. But mine just row by row and block by block comparison tool. Can customize theme.

[u/karthikjpt]

[u/karthikjpt]

[u/Public-Carrot-4485]

the pain point you described is real but pretty niche. before investing in servers and a domain, maybe just ship it as a single HTML file people can download and see if anyone actually uses it consistently

[u/karthikjpt → u/Public-Carrot-4485]

No matter how much I have build best, without experts hands on experience and word of mouth or posting in tech news downloads can't increase